Systems and methods for establishing a diagnostic transmission mode and communicating over the same

ABSTRACT

Upon detection of a trigger, such as the exceeding of an error threshold or the direction of a user, a diagnostic link system enters a diagnostic information transmission mode. This diagnostic information transmission mode allows for two modems to exchange diagnostic and/or test information that may not otherwise be exchangeable during normal communication. The diagnostic information transmission mode is initiated by transmitting an initiate diagnostic link mode message to a receiving modem accompanied by a cyclic redundancy check (CRC). The receiving modem determines, based on the CRC, if a robust communications channel is present. If a robust communications channel is present, the two modems can initiate exchange of the diagnostic and/or test information. Otherwise, the transmission power of the transmitting modem is increased and the initiate diagnostic link mode message re-transmitted to the receiving modem until the CRC is determined to be correct.

RELATED APPLICATION DATA

This application is a continuation of U.S. application Ser. No.12/779,708, filed May 13, 2010, now U.S. Pat. No. 7,889,784, which is acontinuation of 12/477,742, filed Jun. 3, 2009, now U.S. Pat. No.7,835,430, which is a continuation of U.S. application Ser. No.10/619,691, filed Jul. 16, 2003, now U.S. Pat. No. 7,570,686, which is adivisional of U.S. application Ser. No. 09/755,173, filed Jan. 8, 2001,now U.S. Pat. No. 6,658,052, which claims the benefit of and priorityunder 35 U.S.C. §119(e) to U.S. Provisional Application No. 60/224,308,filed Aug. 10, 2000 entitled “Characterization of transmission linesusing broadband signals in a multi-carrier DSL system,” and U.S.Provisional Application No. 60/174,865, filed Jan. 7, 2000 entitled“Multicarrier Modulation System with Remote Diagnostic TransmissionMode”, each of which are incorporated herein by reference in theirentirety.

FIELD OF THE INVENTION

This invention relates to test and diagnostic information. Inparticular, this invention relates to a robust system and method forcommunicating diagnostic information.

BACKGROUND OF THE INVENTION

The exchange of diagnostic and test information between transceivers ina telecommunications environment is an important part of atelecommunications, such as an ADSL, deployment. In cases where thetransceiver connection is not performing as expected, for example, wherethe data rate is low, where there are many bit errors, or the like, itis necessary to collect diagnostic and test information from the remotetransceiver. This is performed by dispatching a technician to the remotesite, e.g., a truck roll, which is time consuming and expensive.

In DSL technology, communications over a local subscriber loop between acentral office and a subscriber premises is accomplished by modulatingthe data to be transmitted onto a multiplicity of discrete frequencycarriers which are summed together and then transmitted over thesubscriber loop. Individually, the carriers form discrete,non-overlapping communication subchannels of limited bandwidth.Collectively, the carriers form what is effectively a broadbandcommunications channel. At the receiver end, the carriers aredemodulated and the data recovered.

DSL systems experience disturbances from other data services on adjacentphone lines, such as, for example, ADSL, HDSL, ISDN, T1, or the like.These disturbances may commence after the subject ADSL service isalready initiated and, since DSL for internet access is envisioned as analways-on service, the effect of these disturbances must be amelioratedby the subject ADSL transceiver.

SUMMARY OF THE INVENTION

The systems and methods of this invention are directed toward reliablyexchanging diagnostic and test information between transceivers over adigital subscriber line in the presence of voice communications and/orother disturbances. For simplicity of reference, the systems and methodsof the invention will hereafter refer to the transceivers generically asmodems. One such modem is typically located at a customer premises suchas a home or business and is “downstream” from a central office withwhich it communicates. The other modem is typically located at thecentral office and is “upstream” from the customer premises. Consistentwith industry practice, the modems are often referred to as “ATU-R”(“ADSL transceiver unit, remote,” i.e., located at the customerpremises) and “ATU-C” (“ADSL transceiver unit, central office” i.e.,located at the central office). Each modem includes a transmittersection for transmitting data and a receiver section for receiving data,and is of the discrete multitone type, i.e., the modem transmits dataover a multiplicity of subchannels of limited bandwidth. Typically, theupstream or ATU-C modem transmits data to the downstream or ATU-R modemover a first set of subchannels, which are usually the higher-frequencysubchannels, and receives data from the downstream or ATU-R modem over asecond, usually smaller, set of subchannels, commonly thelower-frequency subchannels. By establishing a diagnostic link modebetween the two modems, the systems and methods of this invention areable to exchange diagnostic and test information in a simple and robustmanner.

In the diagnostic link mode, the diagnostic and test information iscommunicated using a signaling mechanism that has a very high immunityto noise and/or other disturbances and can therefore operate effectivelyeven in the case where the modems could not actually establish anacceptable connection in their normal operational mode.

For example, if the ATU-C and/or ATU-R modem fail to complete aninitialization sequence, and are thus unable to enter a normal steadystate communications mode, where the diagnostic and test informationwould normally be exchanged, the modems according to the systems andmethods of this invention enter a robust diagnostic link mode.Alternatively, the diagnostic link mode can be entered automatically ormanually, for example, at the direction of a user. In the robustdiagnostic link mode, the modems exchange the diagnostic and testinformation that is, for example, used by a technician to determine thecause of a failure without the technician having to physically visit,i.e., a truckroll to, the remote site to collect data.

The diagnostic and test information can include, for example, but is notlimited to, signal to noise ratio information, equalizer information,programmable gain setting information, bit allocation information,transmitted and received power information, margin information, statusand rate information, telephone line condition information, such as thelength of the line, the number and location of bridged taps, a wiregauge, or the like, or any other known or later developed diagnostic ortest information that may be appropriate for the particularcommunications environment. For example, the exchanged diagnostic andtest information can be directed toward specific limitations of themodems, to information relating to the modem installation and deploymentenvironment, or to other diagnostic and test information that can, forexample, be determined as needed which may aid in evaluating the causeof a specific failure or problem. Alternatively, the diagnostic and testinformation can include the loop length and bridged tap lengthestimations as discussed in U.S. patent application Ser. No. 09/755,172,now U.S. Pat. No. 6,865,221, filed herewith and incorporated herein byreference in its entirety.

For example, an exemplary embodiment of the invention illustrates theuse of the diagnostic link mode in the communication of diagnosticinformation from the remote terminal (RT) transceiver, e.g., ATU-R, tothe central office (CO) transceiver, e.g., ATU-C. Transmission ofinformation from the remote terminal to the central office is importantsince a typical ADSL service provider is located in the central officeand would therefore benefit from the ability to determine problems atthe remote terminal without a truckroll. However, it is to beappreciated, that the systems and the methods of this invention willwork equally well in communications from the central office to theremote terminal.

These and other features and advantages of this invention are describedin or are apparent from the following detailed description of theembodiments.

BRIEF DESCRIPTION OF THE DRAWINGS

The embodiments of the invention will be described in detail, withreference to the following figures wherein:

FIG. 1 is a functional block diagram illustrating an exemplarycommunications system according to this invention; and

FIG. 2 is a flowchart outlining an exemplary method for communicatingdiagnostic and test information according to this invention.

DETAILED DESCRIPTION OF THE INVENTION

For ease of illustration the following description will be described inrelation to the CO receiving diagnostic and test information from theRT. In the exemplary embodiment, the systems and methods of thisinvention complete a portion of the normal modem initialization beforeentering into the diagnostic link mode. The systems and methods of thisinvention can enter the diagnostic link mode manually, for example, atthe direction of a technician or a user after completing a portion ofinitialization. Alternatively, the systems and methods of this inventioncan enter the diagnostic link mode automatically based on, for example,a bit rate failure, a forward error correction or a CRC error duringshowtime, e.g., the normal steady state transmission mode, or the like.The transition into the diagnostic link mode is accomplished bytransmitting a message from the CO modem to the RT modem indicating thatthe modems are to enter into the diagnostic link mode, as opposed totransitioning into the normal steady state data transmission mode.Alternatively, the transition into the diagnostic link mode isaccomplished by transmitting a message from the RT modem to the CO modemindicating that the modems are to enter into the diagnostic link mode asopposed to transitioning into the normal steady state data transmissionmode. For example, the transition signal uses an ADSL state transitionto transition from a standard ADSL state to a diagnostic link modestate.

In the diagnostic link mode, the RT modem sends diagnostic and testinformation in the form of a collection of information bits to the COmodem that are, for example, modulated by using one bit per DTM symbolmodulation, as is used in the C-Rates1 message in the ITU and ANSI ADSLstandards, where the symbol may or may not include a cyclic prefix.Other exemplary modulation techniques include Differential Phase ShiftKeying (DPSK) on a subset or all the carriers, as specified in, forexample, ITU standard G.994.1, higher order QAM modulation (>1 bit percarrier), or the like.

In the one bit per DMT symbol modulation message encoding scheme, a bitwith value 0 is mapped to the REVERB1 signal and a bit with a value of 1mapped to a SEGUE1 signal. The REVERB1 and SEGUE1 signals are defined inthe ITU and ANSI ADSL standards. The REVERB1 signal is generated bymodulating all of the carriers in the multicarrier system with a knownpseudo-random sequence thus generating a wideband modulated signal. TheSEGUE1 signal is generated from a carrier by 180 degree phase reversalof the REVERB1 signal. Since both signals are wideband and known inadvance, the receiver can easily detect the REVERB1 and SEGUE1 signalusing a simple matched filter in the presence of large amounts of noiseand other disturbances.

TABLE 1 Exemplary Message Variables Data Sent in the Diag Link TrainType ADSL Standard Chip Type Vendor ID Code Version Average ReverbReceived Signal Programmable gain amplifier (PGA) Gain - TrainingProgrammable gain amplifier PGA Gain - Showtime Filter Present duringIdle Channel Calculation Average Idle Channel Noise Signal to Noiseduring Training Signal to Noise during Showtime Bits and Gains Data RateFraming Mode Margin Reed-Solomon Coding Gain QAM Usage Frequency DomainEqualizer (FDQ) Coefficients Gain Scale Time domain equalizer (TDQ)Coefficients Digital Echo Canceller (DEC) Coefficients

Table 1 shows an example of a data message that can be sent by the RT tothe CO during the diagnostic link mode. In this example, the RT modemsends 23 different data variables to the CO. Each data variable containsdifferent items of diagnostic and test information that are used toanalyze the condition of the link. The variables may contain more thanone item of data. For example, the Average Reverb Signal contains thepower levels per tone, up to, for example, 256 entries, detected duringthe ADSL Reverb signal. Conversely, the PGA Gain-Training is a singleentry, denoting the gain in dB at the receiver during the ADSL training.

Many variables that represent the type of diagnostic and testinformation that are used to analyze the condition of the link are sentfrom the RT modem to the CO modem. These variables can be, for example,arrays with different lengths depending on, for example, information inthe initiate diagnostic mode message. The systems and methods of thisinvention can be tailored to contain many different diagnostic and testinformation variables. Thus, the system is fully configurable, allowingsubsets of data to be sent and additional data variables to be added inthe future. Therefore, the message length can be increased or decreased,and diagnostic and test information customized, to support more or lessvariables as, for example, hardware, the environment and/or thetelecommunications equipment dictates.

Therefore, it is to be appreciated, that in general the variablestransmitted from the modem being tested to the receiving modem can beany combination of variables which allow for transmission of test and/ordiagnostic information.

FIG. 1 illustrates an exemplary embodiment of the additional modemcomponents associated with the diagnostic link mode. In particular, thediagnostic link system 100 comprises a central office modem 200 and aremote terminal modem 300. The central office modem 200 comprises, inaddition to the standard ATU-C components, a CRC checker 210, adiagnostic device 220, and a diagnostic information monitoring device230. The remote terminal modem 300 comprises, in addition to thestandard components associated with an ATU-R, a message determinationdevice 310, a power control device 320, a diagnostic device 330 and adiagnostic information storage device 340. The central office modem 200and the remote terminal model 300 are also connected, via link 5, to asplitter 10 for a phone switch 20, and a splitter 30 for a phone 40.Alternatively, the ATU-R can operate without a splitter, e.g.,splitterless, as specified in ITU standard G.992.2 (G.lite) or with anin-line filter in series with the phone 40. In addition, the remoteterminal modem 300, can also be connected to, for example, one or moreuser terminals 60. Additionally, the central office modem 200 can beconnected to one or more distributed networks 50, via link 5, which mayor may not also be connected to one or more other distributed networks.

While the exemplary embodiment illustrated in FIG. 1 shows thediagnostic link system 100 for an embodiment in which the remoteterminal modem 300 is communicating test and diagnostic information tothe central office 200, it is to be appreciated that the variouscomponents of the diagnostic link system can be rearranged such that thediagnostic and test information can be forwarded from the central office200 to the remote terminal modem 300, or, alternatively, such that bothmodems can send and receive diagnostic and/or test information.Furthermore, it is to be appreciated, that the components of thediagnostic link system 100 can be located at various locations within adistributed network, such as the—POTS network, or other comparabletelecommunications network. Thus, it should be appreciated that thecomponents of the diagnostic link system 100 can be combined into onedevice for respectively transmitting, receiving, or transmitting andreceiving diagnostic and/or test information. As will be appreciatedfrom the following description, and for reasons of computationalefficiency, the components of the diagnostic link system 100 can bearranged at any location within a telecommunications network and/ormodem without affecting the operation of the system.

The links 5 can be a wired or wireless link or any other known or laterdeveloped element(s) that is capable of supplying and communicatingelectronic data to and from the connected elements. Additionally, theuser terminal 60 can be, for example, a personal computer or otherdevice allowing a user to interface with and communicate over a modem,such as a DSL modem. Furthermore, the systems and method of thisinvention will work equally well with splitterless and low-passmulitcarrier modem technologies.

In operation, the remote terminal 300, commences its normalinitialization sequence. The diagnostic device 330 monitors theinitialization sequence for a failure. If there is a failure, thediagnostic device 330 initiates the diagnostic link mode. Alternatively,a user or, for example, a technician at the CO, can specify that theremote terminal 300 enter into the diagnostic link mode after completinga portion of an initialization. Alternatively still, the diagnosticdevice 330 can monitor the normal steady state data transmission of theremote terminal, and upon, for example, an error threshold beingexceeded, the diagnostic device 330 will initiate the diagnostic linkmode.

Upon initialization of the diagnostic link mode, the diagnostic device330, in cooperation with the remote terminal 300 will transmit aninitiate diagnostic link mode message from the remote terminal to thecentral office 200 (RT to CO). Alternatively, the central office modem200 can transmit an initiate diagnostic link mode message to the remoteterminal modem 300. If the initiate diagnostic link mode message isreceived by the central office 200, the diagnostic device 330, incooperation with the message determination device 310, determines adiagnostic link message to be forwarded to the central office 200. Forexample, the diagnostic link message can include test information thathas been assembled during, for example, the normal ADSL initializationprocedure. The diagnostic and/or test information can include, but isnot limited to, the version number of the diagnostic link mode, thelength of the diagnostic and/or test information, the communicationsstandard, such as the ADSL standard, the chipset type, the vendoridentifications, the ATU version number, the time domain received reverbsignal, the frequency domain reverb signal, the amplifier settings, theCO transmitter power spectral density, the frequency domain receivedidle channel, the signal to noise ratio, the bits and gains and theupstream and downstream transmission rates, or the like.

If the initiate diagnostic link mode message is not received by thecentral office 200, the initiate diagnostic link mode message can, forexample, be re-transmitted a predetermined number of iterations until adetermination is made that it is not possible to establish a connection.

Assuming the initiate diagnostic link mode message is received, then,for a predetermined number of iterations, the diagnostic device 330, incooperation with the remote terminal modem 300 and the diagnosticinformation storage device 340, transmits the diagnostic link messagewith a cyclic redundancy check (CRC) to the central office modem 200.However, it is to be appreciated that in general, any error detectionscheme, such as bit error detection, can be used without affecting theoperation of the system. The central office 200, in cooperation with theCRC checker 210, determines if the CRC is correct. If the CRC iscorrect, the diagnostic information stored in the diagnostic informationstorage device 340 has been, with the cooperation of the diagnosticdevice 330, and the remote terminal modem 300, forwarded to the centraloffice 200 successfully.

If, for example, the CRC checker 210 is unable to determine the correctCRC, the diagnostic device 330, in cooperation with power control device320, increases the transmission power of the remote terminal 300 andrepeats the transmission of the diagnostic link message from the remoteterminal 300 to the central office 200. This process continues until thecorrect CRC is determined by the CRC checker 210.

The maximum power level used for transmission of the diagnostic linkmessage can be specified by, for example, the user or the ADSL serviceoperator. If the CRC checker 210 does not determine a correct CRC at themaximum power level and the diagnostic link mode can not be initiatedthen other methods for determining diagnostic information are utilized,such as dispatching a technician to the remote site, or the like.

Alternatively, the remote terminal 300, with or without an increase inthe power level, can transmit the diagnostic link message several times,for example, 4 times. By transmitting the diagnostic link messageseveral times, the CO modem 200 can use, for example, a diversitycombining scheme to improve the probability of obtaining a correct CRCfrom the received diagnostic link message(s).

Alternatively, as previously discussed, the central office 200 comprisesa diagnostic information monitoring device 230. The remote terminal 300can also include a diagnostic information monitoring device. One or moreof these diagnostic information monitoring devices can monitor thenormal steady state data transmission between the remote terminal 300and the central office 200. Upon, for example, the normal steady statedata transmission exceeded a predetermined error threshold, thediagnostic information monitoring device can initiate the diagnosticlink mode with the cooperation of the diagnostic device 300 and/or thediagnostic device 220.

FIG. 2 illustrates an exemplary method for entering a diagnostic linkmode in accordance with this invention. In particular, control begins instep S100 and continues to step S110. In step S110, the initializationsequence is commenced. Next, in step S120, if an initialization failureis detected, control continues to step S170. Otherwise, control jumps tostep S130. In step S130, a determination is made whether the diagnosticlink mode has been selected. If the diagnostic link mode has beenselected, control continues to step S170, otherwise, control jumps tostep S140.

In step S170, the initiate diagnostic link mode message is transmittedfrom, for example, the remote terminal to the central office. Next, instep S180, a determination is made whether the initiate diagnostic modemessage has been received by the CO. If the initiate diagnostic modemessage has been received by the CO, control jumps to step S200.Otherwise, control continues to step S190. In step S190, a determinationis made whether to re-transmit the initiate diagnostic mode message, forexample, based on whether a predetermined number of iterations havealready been completed. If the initiate diagnostic mode message is to bere-transmitted, control continues back to step S170. Otherwise, controljumps to step S160.

In step S200, the diagnostic link message is determined, for example, byassembling test and diagnostic information about one or more of thelocal loop, the modem itself, the telephone network at the remoteterminal, or the like. Next, in step S210, for a predetermined number ofiterations, steps S220-S240 are completed. In particular, in step S220 adiagnostic link message comprising a CRC is transmitted to, for example,the CO. Next, in step S230, the CRC is determined. Then, in step S240, adetermination is made whether the CRC is correct. If the CRC is correct,the test and/or diagnostic information has been successfullycommunicated and control continues to step S160.

Otherwise, if step S210 has completed the predetermined number ofiterations, control continues to step S250. In step S250, thetransmission power is increased and control continues back to step S210.Alternatively, as previously discussed, the diagnostic link message maybe transmitted a predetermined number of times, with our without achange in the transmission power.

In step S140, the normal steady state data transmission is entered intobetween two modems, such as the remote terminal and the central officemodems. Next, in step S150, a determination is made whether an errorthreshold during the normal steady state data transmission has beenexceeded. If the error threshold has been exceeded, control continues tostep S170. Otherwise, control jumps to step S160. In step S160, thecontrol sequence ends.

As shown in FIG. 1, the diagnostic link mode system can be implementedeither on a single program general purpose computer, a modem, such as aDSL modem, or a separate program general purpose computer having acommunications device. However, the diagnostic link system can also beimplemented on a special purpose computer, a programmed microprocessoror microcontroller and peripheral integrated circuit element, an ASIC orother integrated circuit, a digital signal processor, a hardwiredelectronic or logic circuit such as a discrete element circuit, aprogrammed logic device such as a PLD, PLA, FPGA, PAL, or the like, andassociated communications equipment. In general, any device capable ofimplementing a finite state machine that is capable of implementing theflowchart illustrated in FIG. 2 can be used to implement a diagnosticlink system according to this invention.

Furthermore, the disclosed method may be readily implemented in softwareusing object or object-oriented software development environments thatprovide portable source code that can be used on a variety of computer,workstation, or modem hardware platforms. Alternatively, the discloseddiagnostic link system may be implemented partially or fully in hardwareusing standard logic circuits or a VLSI design. Other software orhardware can be used to implement the systems in accordance with thisinvention depending on the speed and/or efficiency requirements of thesystems, the particular function, and a particular software or hardwaresystems or microprocessor or microcomputer systems being utilized. Thediagnostic link system and methods illustrated herein however, can bereadily implemented in hardware and/or software using any known or laterdeveloped systems or structures, devices and/or software by those ofordinary skill in the applicable art from the functional descriptionprovided herein and with a general basic knowledge of the computer andtelecommunications arts.

Moreover, the disclosed methods can be readily implemented as softwareexecuted on a programmed general purpose computer, a special purposecomputer, a microprocessor, or the like. In these instances, the methodsand systems of this invention can be implemented as a program embeddedon a modem, such a DSL modem, as a resource residing on a personalcomputer, as a routine embedded in a dedicated diagnostic link system, acentral office, or the like. The diagnostic link system can also beimplemented by physically incorporating the system and method into asoftware and/or hardware system, such as a hardware and software systemsof a modem, a general purpose computer, an ADSL line testing device, orthe like.

It is, therefore, apparent that there is provided in accordance with thepresent invention, systems and methods for transmitting a diagnosticlink message. While this invention has been described in conjunctionwith a number of embodiments, it is evident that many alternatives,modifications and variations would be or are apparent to those ofordinary skill in the applicable arts. Accordingly, applicants intend toembrace all such alternatives, modifications, equivalents and variationsthat are within the spirit and the scope of this invention.

What is claimed is:
 1. A system comprising: a transceiver operable tocomplete one or more portions of an initialization sequence; and adevice, in communication with the transceiver, configurable to cause thetransceiver to enter into a diagnostic mode when the one or moreportions of the initialization sequence are not successfully completed,wherein the transceiver is also operable to transmit at least onemessage during the diagnostic mode and wherein the at least one messagehas a higher immunity to noise than at least one message transmittedduring the one or more portions of the initialization sequence.
 2. Thesystem of claim 1, wherein the transceiver is further operable totransmit a message to enter the diagnostic mode.
 3. The system of claim1, wherein the transceiver is further operable to receive a messagecomprising one or more of a signal to noise ratio per subchannel, afrequency domain idle channel noise per subchannel, and a power levelper subchannel.
 4. The system of claim 3, wherein the message isreceived a plurality of times.
 5. The system of claim 1, wherein thetransceiver includes at least one digital signal processor.
 6. Thesystem of claim 1, wherein the transceiver includes at least one ASIC(application specific integrated circuit).
 7. The system of claim 1,wherein the transceiver is operable to transmit internet access data. 8.The system of claim 1, wherein the transceiver is a central officetransceiver.
 9. The system of claim 1, wherein the transceiver is acustomer premises equipment.
 10. A method for communicating diagnosticinformation in a telecommunication network comprising: configuring adevice, in communication with a transceiver, to cause the transceiver toenter into a diagnostic mode when one or more portions of aninitialization sequence is not successfully completed; and transmitting,by the transceiver, at least one message during the diagnostic modewherein the at least one message has a higher immunity to noise than atleast one message transmitted during the one or more portions of theinitialization sequence.
 11. The method of claim 10, further comprisingtransmitting a message to enter the diagnostic mode.
 12. The method ofclaim 10, further comprising receiving a message comprising one or moreof a signal to noise ratio per subchannel, a frequency domain idlechannel noise per subchannel, and a power level per subchannel.
 13. Themethod of claim 12, wherein the message is received a plurality oftimes.
 14. The method of claim 10, wherein the transceiver is installedin a central office.
 15. The method of claim 10, wherein the transceiveris located in a customer premises.
 16. The method of claim 10, whereinthe transceiver is installed in a central office and a secondtransceiver located in a customer premises and the transceivers areconnected using a telephone line that experiences disturbances fromother data services on adjacent telephone lines.
 17. The method of claim16, wherein the telephone line is used to transport internet accessdata.
 18. The method of claim 16, wherein a service provider determinesproblems on the telephone line premises without a truck roll.
 19. Atelecommunications system comprising: a first transceiver connected to atelephone line that experiences disturbances from other data services onadjacent telephone lines; a second transceiver connected to thetelephone line and located in a customer premises; and a device, incommunication with the first transceiver, configurable to cause thefirst transceiver to enter into a diagnostic mode when one or moreportions of an initialization sequence are not successfully completed,wherein at least one message transmitted during the diagnostic mode hasa higher immunity to noise than at least one message transmitted duringthe one or more portions of the initialization sequence.